The F-plasmid protein CcdB kills E. coli cells that express it, unless they also express its inhibitor CcdA. CcdB acts by inhibiting DNA gyrase and causing it to cleave chromosomal DNA. We have studied the interaction of CcdB with DNA gyrase. We found that there are two different complexes between CcdB protein and gyrase. One complex (probably the lethal one) causes gyrase to cleave DNA, but does not appreciably inhibit supercoiling. The other complex inhibits supercoiling but does not cleave DNA. This second complex can be isolated from cells, but its formation in vitro requires the CcdB and GyrA proteins to be denatured and then renatured together. Both complexes appear to involve the same functional groups on CcdB and GyrA. These results are now being compared to crystallographic data on the GyrA and CcdB proteins that are becoming available.